The present invention relates to a method and apparatus for forming an electrically conductive layer on a corrosion-resistant layer formed of plastic material such as polyethylene (PE) or polyvinyl chloride (PVC) which covers a cable.
The corrosion-resistant layer of a cable sheathed with metal such as lead or aluminum is, in general, electrically insulating. The corrosion-resistant layer serves not only to protect the metal sheath mechanically but also to prevent electrical or chemical corrosion of the metal sheath. If, during the use of the cable, the corrosion-resistant layer becomes defective, then the metal sheath may corrode by action of moisture entering through the defective corrosion-resistant layer. If the reaction is advanced, moisture may enter the inside of the cable through the metal sheath and decrease the insulation resistance of the cable. Therefore, it is desirable during the use of the cable to frequently inspect the corrosion-resistant layer to determine whether its structure is satisfactory.
In order to simplify this inspection, heretofore a method was employed utilizing a specific type of cable in which during the manufacture of the cable an electrically conductive layer is formed on the electrically insulating corrosion-resistant layer of the cable. With the electrically conductive layer formed on the corrosion-resistant layer, then the acceptability of the corrosion-resistant layer over the entire length of the cable can be determined by applying a voltage across the metal sheath and the electrically conductive layer.
In order to form the electrically conductive layer on the corrosion-resistant layer, heretofore the following method was employed. For a cable having a PVC corrosion-resistant layer, carbon powder or graphite powder was mixed with PVC powder to obtain an electrically conductive powder. The electrically conductive powder thus prepared was dissolved in a solvent such as methyl-ethyl-ketone to make an electrically conductive paint. The electrically conductive paint thus prepared was put in a paint tank and the cable passed through the electrically conductive paint to coat the cable with the paint. Thereafter, the solvent was allowed to evaporate to complete the formation of the electrically conductive layer on the PVC corrosion-resistant layer.
However, the conventional method is disadvantageous in the following points. It is difficult to form a uniform electrically conductive layer and accordingly, an electrically conductive layer actually formed has a tendency to peel off the cable. As the PVC powder is dissolved with a solvent, the corrosion-resistant layer is liable to crack. Furthermore, it is considerably troublesome to control the concentration of the electrically conductive paint and the working conditions. In addition, the solvent is hazardous and has a foul order which may make the environmental working conditions unsatisfactory. Moreover, the conventional method cannot be used with a PE corrosion-resistant layer because there is no suitable solvent therefor.
For a cable having a PE corrosion-resistant layer as described above, a method has been proposed in the art in which the cable is passed through electrically conductive PE powder to form an electrically conductive layer on the corrosion-resistant layer. However, the conventional method is disadvantageous in that it is rather difficult to make the powder adhere to the corrosion-resistant layer because its adhering properties are generally insufficient and it is still impossible with this method to form a uniform electrically conductive layer.
Accordingly, an object of the invention is to provide an electrically conductive layer which is uniform and which firmly adheres to the cable and which will not peel off the cable by employing a method in which electrically conductive powder mixed with binder powder is adhered to the surface of the corrosion-resistant layer of the cable then pressurized and heated so that it is firmly attached to the corrosion-resistant layer whereby the above-described drawbacks accompanying a conventional method are eliminated.